Polymer stream transfer

The invention relates to apparatus for improving the degassing of polymers, in particular olefin polymers.

[0002] The polymerization of olefins, during which the olefin monomer and optional olefin comonomer are polymerized, usually in the presence of a catalyst and / or solvent, is commonly known. The polymer is withdrawn from the polymerization reactor together with reactants and inert hydrocarbons. Reagents and hydrocarbons should be recovered for economic, safety and environmental reasons, and many processes are known in the prior art to achieve this goal. These processes essentially include reducing the pressure and degassing the polymer-containing stream after withdrawing it from the polymerization reactor. The requirement for degassing is most critical in processes involving polymers withdrawn from a reactor with a high content of absorbed or free liquid hydrocarbons.

[0003] Over the years, there has been a steady increase in the maximum production scale of industrial installations, and as the production pace increased, the impact of potential disruptions at the manufacturing process has also increased considerably, affecting not only the polymers themselves, but also the devices before and after the process. At the same time, the growing process experience has led to the processing of increasingly high concentrations of solid particles (loads) of the polymer withdrawn from the reactors. The increase in solids concentrations in slurry polymerization devices was usually achieved by increased circulation rates, which were obtained, for example, due to the higher energy requirements of the reactor as described in the patents EP 432555 and EP 891990. The increase in solid charge is desirable to increase the residence time in the reactor at a constant reactor volume, as well as to reduce the requirements for purifying and recycling the solvent after passing through the reactor. However, the flow of products at high stresses with solid particles is more problematic, which requires careful design and process experience to avoid blocking and polymer blocking problems that do not occur at lower solids loads.

[0004] During and as a result of the process of pressure reduction and degassing of the polymer stream withdrawn from the polymerization reactor, the temperature of the polymers decreases. It is well known that the degassing process and desorbing of the polymer is significantly improved by keeping the polymer temperature as high as possible. Thus, in slurry loop processes, the transfer line between the polymerization reactor and the decompression (degassing) tank for the polymeric stream is usually

- 2 heated. As an example of a typical process, in the patents WO04 / 031245 and WO 05/044871, the reception line from a loop polymerization reactor includes a flashing line comprising a withdrawn slurry surrounded by a conduit into which a heated liquid such as low pressure steam is supplied. , to provide an indirect heating of the slurry. However, it is also well known that the viscosity and susceptibility of the flowing polymer to the deposition and / or plugging of transfer lines and reservoirs generally increases with temperature and plugging or deposition problems become more severe as the solid particle load increases in the transfer system used according to above.

[0005] The degassing of the polymer stream causes the vaporization of the liquid phase of the stream, which results in an increase in the volume on the transfer line and consequently an increase in the speed of the jet. However, if the speed is too high, it may exceed the sound speed (speed of sound propagation in a given substance) leading to the flow interruption. On the other hand, if the initial velocity is too low, there is an increased risk of clogging or deposition of the solid polymer as mentioned above.

[0006] In large plants, the transfer line must be very long to allow sufficient heating, such length may affect the spatial planning of the plant. This can cause various problems related to the space occupied by the equipment in the plant and the control of the conditions inside the transfer line. It is often necessary to heat a significant part of the transfer line length to meet the heat load requirements. Thus, it should be appreciated that making the polymer stream at the desired temperature and pressure flow to the degassing tank with minimal clogging / deposition is a major technical challenge.

[0007] The invention aims to optimize the heating of the polymer during its flow from the reactor to the degassing tank and thus to maintain the undisturbed flow of product due to the specific design of the transfer line between the polymerization reactor and the degassing tank for the polymer stream.

[0008] Thus, in a first aspect, the invention provides a method of heating a stream comprising a polymer flowing from a polymerization reactor to a degassing tank in accordance with claim 1.

[0009] It should also be appreciated that the polymer-containing stream continuously flows through some of the pipelines from the moment it leaves the polymerization reactor and before it enters the degassing tank. For the purposes of the invention, the heater comprises a portion of the pipeline from the beginning of the heated section (or the first of the heated sections) to the end of the heated section (or to the last of the heated sections). In this context, the term "heater" used below includes within its scope a plurality of possible heaters connected in series. Heating outlet (or heater transfer line)

- 3 is at the end of the heated section of the line and the inlet of the heater is at the beginning of the heated section of the line, the heated section of the line comprising a single heater or a plurality of heaters in a series. The term "line" means any form of a duct suitable for transporting a stream containing a polymer containing solid, liquid and gas. Thus, the speeds Vi and Vo are average speeds at the inlet and outlet of the transfer line / heater, wherein "inlet" and "outlet" should be understood in accordance with the above definition. The term "average speed" means the average speed over the entire cross section of the stream at any point along the length of the transfer line, such as an outlet or inlet.

[0010] For the method according to the invention, it is important to maintain an average speed of the polymer-containing stream at a sufficient level to avoid the risk of clogging or blocking. It has been found that the transfer line design according to the invention enables reliable operation while controlling the Vo / Vi ratio within favorable limits. Typical Vi values ​​are 3-20 m / s, and typical Vo values ​​are 3080 m / s.

It is advantageous if the stream transfer line is at least 20m in length and when the temperature of the stream containing the polymer at the outlet from the heater exceeds the dew point of the stream, and the temperature of the stream along the transfer line is kept below the softening temperature of the polymer, the softening point of the polymer is defined as the Vicat softening point according to ASTM D1525, ISO 306 at 10N load.

[0012] Preferably, the temperature of the stream containing the polymer at the outlet of the heater is 5-80 ° C, most preferably 10-30 ° C, above the dew point of the stream.

[0013] Furthermore, it is preferred that the temperature of the inner surface of the transfer line along its length is maintained below the softening point of the polymer.

[0014] The polymer-containing stream is preferably withdrawn from the polymerization reactor prior to entering the heater, the inlet of the heater is thus directly connected to the polymerization reactor. Furthermore, the outlet of the heater is preferably directly connected to a degassing tank, which is usually placed before the last polymer treatment and extrusion or before the next polymerization reactor. The stream may be continuously withdrawn from the polymerization reactor and may contain an active polymer.

[0015] The Vicat softening temperature according to ASTM D1525, ISO 306 is the temperature at which the flattened needle penetrates the polymer sample to a depth of 1 mm at a load of 10N. The temperature reflects the expected softening point when the material is used in applications at elevated temperatures. A test sample that has a thickness between 3mm and 6.5mm, and a width and length of at least 10mm is placed in the test apparatus (e.g. ROSAND ASP 6 HDT / VICAT System) so that the penetrating needle, which has a cross-sectional area at its tip equal to 1mm², rests on the surface of the sample at least 1mm from the edge. The sample was applied

- 4 load of 10N. The sample is then lowered into an oil bath at 23 ° C. The bath temperature is raised at 50 ° C per hour until the needle penetrates 1mm; the temperature at which this will happen is Vicat's softening temperature.

[0016] The pressure drop across the transfer line per unit length is preferably between 0.01 bar / m and 0.2 bar / m, preferably between 0.0125 bar / m and 0.1 bar / m, most preferably between 0.0125 bar / m 0.04 bar / m. A typical pressure at the inlet of the heater, Pi, is 530 bar, preferably 10-25 bar. The outlet pressure is typically 1.5-12 bar, preferably 7-11 bar.

[0017] In the case where the heater is placed between the polymerization reactor and the degassing tank, the pressure drop in the heater is usually between 5% and 50%, preferably between 10 and 35% of the total pressure drop between the polymerization reactor and the entrance to the degassing tank.

[0018] By maintaining the temperature of the stream containing the polymer at the heater outlet above the dew point of the stream and the inner surface of the transfer line below the softening point of the polymer, it is possible to ensure that all of the liquid in the stream is evaporated until the stream reaches the exit from the heater while simultaneously minimizing the risk of clogging. In the case where the heater is placed between the polymerization reactor and the degassing tank, the temperature of the inner surface of the transfer line can be maintained above the reactor temperature. For a polymer with a density of 935-945 kg / m³ the sidewall temperature of the process is usually controlled between 75 and 130 ° C, preferably between 85 and 105 ° C. For a polymer having a density of 955-965 kg / m³ the wall temperature on the process side is usually controlled between 80 and 135 ° C, preferably between 95 and 110 ° C.

The temperature of the outlet of the transfer line, in particular the temperature of the inner wall of the line at its outlet immediately below the last heated part, is preferably maintained at a level above the temperature of the inner wall at the inlet directly above the first heated portion, more preferably at least 5 ° C higher than the inlet temperature.

[0020] For the method according to the invention, it is highly desirable to maintain an average speed of the polymer-containing stream at a level high enough to avoid the risk of blockages or blockages. The term "mean" means the mean velocity over the entire flow cross-section at any point along the length of the transfer line. Therefore, it is advantageous if the average speed at the inlet Vi is at least 2 m / s, preferably at least 5 m / s and most preferably at least 8 m / s. In addition, it is desirable to keep the speed below the sound speed. Thus, the average velocity at the outlet of Vo is preferably less than 80 m / s, preferably less than 70 m / s. Preferably, Vo is at least 20 m / s. The ratio of average muzzle velocity to average inlet velocity (outlet and inlet of the transfer line as previously defined),

- 5 between 1.5 and 4.

In the method according to the invention, in the case where the heater is placed between the polymerization reactor and the degassing tank, the polymer-containing stream is heated so that at least 90 mol%, preferably at least 98 mol% and optimally 100 mol% of the hydrocarbon fluid withdrawn during the operation of the polymerization reactor it evaporates before entering the degassing tank. The degassing tank operates at a pressure between 6 bar and 12 bar, while maintaining a pressure drop over the entire length of the heater below 0.5 bar per ton of polymer, most preferably between 0.1 bar / ta 0.3 bar / t. It has been found that with such an optimized low pressure drop per production unit, continuous operation is possible even at high loads of solid particles at the entrance to the heater. The solids content in the polymer-containing stream is preferably between 35 wt.%. and 70 wt.%, most preferably between 50 wt.%. and 65% by weight, when the stream flows into the heater, and it is further advantageous if the flow rate at the entry to the heater does not differ by more than 15%, preferably not more than 5% in any of the 30-second periods. One of the ways in which this can be achieved is the use of continuous rather than intermittent reception from the polymerization reactor. This action at high loads with solid particles combined with the increasing diameter of the heater makes it possible to minimize the pressure drop in the heater. when the flow rate at the entry to the heater does not differ by more than 15%, preferably not more than 5% in any of the 30-second periods. One of the ways in which this can be achieved is the use of continuous rather than intermittent reception from the polymerization reactor. This action at high loads with solid particles combined with the increasing diameter of the heater makes it possible to minimize the pressure drop in the heater. when the flow rate at the entry to the heater does not differ by more than 15%, preferably not more than 5% in any of the 30-second periods. One of the ways in which this can be achieved is the use of continuous rather than intermittent reception from the polymerization reactor. This action at high loads with solid particles combined with the increasing diameter of the heater makes it possible to minimize the pressure drop in the heater.

[0022] The average Reynolds number in the cross section of the stream at any point along the length of the heater transfer line should always be greater than 500,000, preferably between 1 million and 10 million, most preferably between 1.8 million and 5 million.

[0023] Heating and reducing the pressure of the polymer stream along with its flow along the transfer line to the degassing tank causes progressive evaporation of the liquid in the stream and consequently an increase in velocity along the line. During the degassing of the transfer line there is a need to meet conflicting requirements to ensure an effective and uninterrupted flow of polymers and heat. Although high speeds increase heat transfer and substantially minimize clogging, they also lead to high pressure drops along the line. It is therefore important to be able to minimize the length of the transfer line and the required heat flow surface while obtaining sufficiently degassed polymer at an acceptable temperature.

[0024] Regarding the construction of the heater itself, preferably the ratio of the outlet diameter of the transfer line To to its inlet diameter Di, To / Di, is greater than 1, preferably between 1.2 and

10. It is usually at least 1.3, and commonly at least 1.4. However, this ratio is preferably not more than 4, and more preferably no more than 2, with the maximum most favorable value of 1.9. It has been found that the increasing diameter of the transfer line along its length allows the heater to adopt a wider range of the flow velocity of the polymer-containing stream. The relatively small inlet diameter allows a relatively high speed even at low flow rates, reducing this

- the risk of clogging itself, while the relatively larger outlet diameter avoids the risk of speeding up the speed of sound even at high flow rates. Having such a bandwidth of bandwidth is particularly valuable during start-up and shut-down operations. To reduce the risk of blockages below the heater, it is also advantageous if the diameter of the outlet To the transfer line is smaller than the outlet of the solid particles of the degassing tank. To is the inner diameter of the transfer line at its outlet and Di is the internal diameter of the transfer line at its inlet, the inlet and outlet of the transfer line being defined earlier.

[0025] The inner diameter D of the transfer line preferably has at least 20mm in length and more usually between 40mm and 200mm. The most preferred are internal diameters between 60mm and 150mm.

[0026] The length L of the heater, and thus the transfer line, is preferably at least 20 m, more preferably at least 30 m but is usually not more than 600 m. A preferred length range is from 50m to 500m, more preferably from 70m to 300m.

[0027] The ratio of the length L of the transfer line to its average internal diameter Dave, L / Dave is preferably from 500 to 10,000, more preferably from 1500 to 3500 and most preferably from 2000 to 3000. If the transfer line is made up of a plurality of sections, each has a different diameter, Dave is the average inner diameter of these sections weighted by the length of each segment; alternatively it can be calculated by reference to the total internal volume V of the line, where V = (π Dave².L) / 4.

[0028] If the diameter of the transfer line increases along its length, this increase preferably takes place at individual stages and not continuously. Usually there is one, two or three lengths of diameter along the length of the pipe.

[0029] Preferably, one or all of the line sections are substantially vertical rather than horizontally mounted such that the line takes up a smaller surface in the plant: in this configuration, the first line segment preferably has its inlet at the bottom so that the initial material flow through the transfer line is flow up. Preferably, less than 20%, most preferably less than 10% of the length of the transfer line is horizontal and optimally the line is constructed essentially without horizontal sections. In one embodiment, at least the inlet and exit of the heated transfer line are oriented vertically so that the inlet flow through the line is upward flow and the outlet flow from the line is downward flow. In one embodiment of the invention, the transfer line comprises a plurality of sections connected by bends (elbows), which are usually U-shaped so that the line returns once or repeatedly. The advantage of this configuration is the smaller size of the transfer line in the plant. Episodes between the elbows are usually straight. The bends can be heated like the rest of the line, but usually, to simplify the construction of the heater, they are not heated. It is generally advantageous if any extension of the line diameter takes place in the unheated section of the line; therefore, line segments may have different diameters with an increase in diameter occurring on one or more elbows, preferably at the elbow exit, so that the velocity drops at the exit from the elbow and not at its inlet, and most preferably at the exit at the top Episodes between the elbows are usually straight. The bends can be heated like the rest of the line, but usually, to simplify the construction of the heater, they are not heated. It is generally advantageous if any extension of the line diameter takes place in the unheated section of the line; therefore, line segments may have different diameters with an increase in diameter occurring on one or more elbows, preferably at the elbow exit, so that the velocity drops at the exit from the elbow and not at its inlet, and most preferably at the exit at the top Episodes between the elbows are usually straight. The bends can be heated like the rest of the line, but usually, to simplify the construction of the heater, they are not heated. It is generally advantageous if any extension of the line diameter takes place in the unheated section of the line; therefore, line segments may have different diameters with an increase in diameter occurring on one or more elbows, preferably at the elbow exit, so that the velocity drops at the exit from the elbow and not at its inlet, and most preferably at the exit at the top

- 7 vertically heated section. The design of elongated sections and bends in the transfer line is crucial for reliable operation without clogging. The number of vertical or horizontal sections between the elbows forming the whole transfer line can be from 2 to 10, although there are more commonly 3 to 7 sections.

[0030] The elbows of the transfer line may have different degrees of curvature. The radius of curvature determined by the elbow can be expressed as a multiple of the diameter D of the line at this point. Elbows usually have radii between 3D and 30D, with 5D-20D being the most favorable to ensure reliable operation without clogging while minimizing the space occupied by the line. As previously stated, the elbows are preferably U-shaped, although alternative options, such as L-shaped elbows, that allow an undisturbed flow path are not excluded. Of course, combinations of the above types of elbows or elbows with different angles such as 60 ° or 120 ° can be used in the transfer line created in sections.

[0031] It has been found that the length of any elongated section of the transfer line should be greater than 0.25D, preferably between 0.5D and 10D, most preferably between 0.75D and 3D. Each elongated section is preferably located directly above or below the elbow, preferably directly below the elbow. It is also advantageous if the elongation is concentric, although other elongation shapes are also possible.

The total heat transfer surface of the transfer line, which is its outer surface in contact with the heating means, is at least 0.5 m² heat exchange surface area per ton / h of polymer production, usually between 0.7 and 10, more preferably between 1 and 5, most preferably between 1.5 and 3.5 heat exchange surfaces per ton / h of polymer production.

Preferably the inlet of the heater is at approximately the same height as the exit from the polymerization reactor to which it is connected, preferably the transfer line from the polymerization reactor to the inlet of the heater is substantially horizontal.

[0034] Most preferably, the exit from the transfer line (at the entry point to the degassing tank) is at a higher elevation than the inlet of the transfer line and / or the outlet from the polymerization reactor.

[0035] The means for heating the transfer line typically comprise a skirt surrounding the line. The heating jacket may be in the form of an electric heater, but it preferably has the form of a concentric tube surrounding the line through which the heating fluid flows. The most commonly used heating fluid is water vapor. It has been found that the conditions can be optimized by using chilled water vapor as a heating medium, especially when the maximum saturation temperature is 0-30 ° C and preferably no more than 10 ° C below the softening point of the polymer being heated. Regardless of the form, the lagging can provide the same heat load over the entire length of the transfer line or it can provide differential heating on different parts of the line. In addition, it is possible that parts of the line (such as bends) remain unheated, as described above. It was found that the optimal thermal load on the length of the line

- Transfer 8 is achieved with a design such that the temperature of the heating medium (or the temperature of the inner wall of the line) is higher at the inlet to the line than at the exit from it. Thus, with the increase of the vapor fraction in the stream containing the polymer during its flow through the line, it is advantageous if the temperature of the heating medium (or the temperature of the inner wall of the line) decreases. This can be achieved in a step-wise manner or through a number of separate steps using sections of different temperatures. However, the most advantageous is the lagging, which operates at different temperatures on different parts of the line, usually due to the independent supply of heating medium for each section where different temperature is required.

[0036] In a preferred embodiment of the invention, the transfer line is heated by a concentric pipe using steam as the heating medium. The temperature of the transfer line outlet is preferably controlled depending on the steam flow rate: for a given steam temperature this has the advantage of allowing control of the transfer line wall temperature to provide a lower temperature at a low polymer flow rate and a higher temperature for a high flow rate when the velocities are higher.

[0037] One way to further increase the temperature of the polymer-containing stream itself at the outlet of the transfer line (apart from increasing the energy supply to the heater) is to increase the solids content in the stream. This can be done by increasing the solids content of the stream withdrawn from the polymerization reactor and / or by using a device for increasing the concentration of particulates above the transfer line. The particulates can carry more heat than the liquid or gaseous components of the stream, therefore they require a lower insert from the transfer line heater to achieve the desired temperature.

[0038] The use of above the transfer line of the solid-concentration enhancement apparatus with prior solvent washing (described in patent EP1118624) is a preferred embodiment of the invention and makes it possible to minimize the concentration of monomers on the transfer line, thereby reducing the risk of clogging.

[0039] The pipe is preferably easily detachable along the length of the heater to facilitate cleaning. Preferably, the pipe is covered by a collar at intervals of 5-15m. If the heating is carried out by means of a jacket with a heating fluid, preferably the heating liquid does not cover any flanges.

[0040] To maximize thermal exchange into the polymer-containing stream, the pipe is preferably made of a material having a thermal conductivity greater than 30 Wm.^-2K¹preferably greater than 40 Wm^-2K^-1. The pipe is usually seamless, although the weld seam pipe is advantageous when a large amount of heat is required.

[0041] Preferably, all of the polymer-containing stream exiting the polymerization reactor is passed through a single transfer line, especially at start-up or at times when the exit speed from a single heater is less than 50 to 60 m / s. Such a transfer line can be powered by one or more

- 9 withdrawing lines from the reactor. The withdrawn stream from the reactor can be concentrated, preferably by means of gravitational or centrifugal means, most preferably by means of a hydrocyclone, before passing through the transfer line. However, it is also within the scope of the invention to provide a plurality of parallel transfer lines receiving a polymer-containing stream, each of which is arranged according to the invention. Thus, a further aspect of the invention provides a method of heating a stream comprising a polymer flowing from a polymerization reactor increasing its production rate to a degassing tank, which comprises

a) flow through the first heater or heaters, each comprising a transfer line for the stream and means for heating the transfer line and increasing the flow rate of the stream preferably while maintaining the temperature of the polymer-containing stream at the outlet from each heater above the dew point of the stream and maintaining the stream temperature at any point along the length of each transfer line below the softening point of the polymer, then

b) flow of a portion of the stream through an additional heater arranged parallel to the first heater, the additional heater also includes a transfer line for the stream and means for heating the transfer line, preferably maintaining the temperature of the polymeric stream leaving all of the above heaters above the dew point of the stream and maintaining the temperature the stream at any point along the length of all transfer lines below the softening point of the polymer.

[0042] A related aspect of the invention provides a method of heating a stream comprising a polymer flowing from a polymerization reactor to a degassing tank, which comprises

a) flow through at least two heaters placed parallel to each other, each comprising a transfer line for the stream and means for heating the transfer line, preferably maintaining the temperature of the polymer-containing stream at the outlet from each heater above the dew point of the stream and maintaining the stream temperature at any point along the length of each transfer line below the softening point of the polymer and decreasing the flow rate of the stream until the exit speed from the heater drops below 40 m / s, and then

b) switching off one of the heaters and passing the stream only through the remaining heater or heaters.

In an alternative embodiment, a portion of the stream is discharged through an additional heater arranged parallel to the first heater if any of the Vo / Vi speeds drops to the desired minimum or below (usually 0.8, preferably 1.3) or alternatively if the pressure drop over of the transfer line per unit length will exceed the desired maximum (typically 0.2 bar, preferably 0.1 bar), the additional heater also includes a transfer line for the stream and means for heating the transfer line. Also in this embodiment, the temperature of the containing stream

- the polymer at the outlet from all heaters is preferably kept above the dew point of the stream and the temperature of the stream at any point along the length of all transfer lines is kept below the softening point of the polymer.

[0044] In such a parallel embodiment of the heater according to the invention, it is not necessary for all transfer lines to operate at any time. In a further embodiment, the polymerization reactor has a plurality of retraction lines, each of which has its own transfer line. The invention also includes within its scope the use of single or parallel devices increasing the concentration of solid particles, traditionally one device is added to increase the concentration of particulates above each transfer line.

[0045] In a parallel embodiment of the heater, it is preferred that at least two heaters operate, the average flow speed at any cross-section of each heater transfer line is maintained between 2 and 100 m / s, most preferably between 10 and 70 m / s.

[0046] The performance of each transfer line can be monitored using parameters including steam flow into the heating envelope or steam valve position for measuring the heat input (load) delivered to the stream, the difference between the pressure in the heater and the output of the reactor pressure valve to measure the flow or the flow rate to each transfer line, the ratio between the steam flow and the outlet temperature for each heater, the mass balance of the reactor to calculate the total flow to all heaters and the difference between the temperature of the heater outlet vapor and the dew point of the stream being processed. The pressure drops on the transfer lines of each heater are preferably the same as in the above-described embodiment with a single heater.

[0047] In the parallel operation of more than one transfer line (heater), minor differences in installation conditions, operation of transfer lines and related equipment above and below these lines, as well as in the pipe system may result in unequal suspension flows (loads) in the transfer lines. If no corrections are made, this may result in a more frequent need to start or stop transfer lines to keep each one within the appropriate working range. To avoid this problem, it is advantageous to automatically balance the slurry load on each transfer line by first determining the slurry flow through each transfer line and then calculating the average flow to all transfer lines, and then applying a bias to the control valve above each transfer line to adjust the feed rate and average the slurry load value on each transfer line. This is preferably achieved by applying a water vapor flow to the envelope surrounding each transfer line (i.e., the amount of heat applied to each transfer line) as a means to determine the slurry flow through each transfer line because the steam flow is controlled to obtain the desired temperature at the outlet of the line. transfer temperature, and the outlet temperature is determined by the flow of slurry through each transfer line. Then, the average is calculated This is preferably achieved by applying a water vapor flow to the envelope surrounding each transfer line (i.e., the amount of heat applied to each transfer line) as a means to determine the slurry flow through each transfer line because the steam flow is controlled to obtain the desired temperature at the outlet of the line. transfer temperature, and the outlet temperature is determined by the flow of slurry through each transfer line. Then, the average is calculated This is preferably achieved by applying a water vapor flow to the envelope surrounding each transfer line (i.e., the amount of heat applied to each transfer line) as a means to determine the slurry flow through each transfer line because the steam flow is controlled to obtain the desired temperature at the outlet of the line. transfer temperature, and the outlet temperature is determined by the flow of slurry through each transfer line. Then, the average is calculated and the outlet temperature is determined by the flow of the slurry through each transfer line. Then, the average is calculated and the outlet temperature is determined by the flow of the slurry through each transfer line. Then, the average is calculated

- a flow of water vapor to the transfer lines, and then a preload is applied to the valve above each transfer line to adjust the flow rate to average the load value on each transfer line measured by the flow of steam. The main control function of the supply valves on all transfer lines is to control the reactor pressure by controlling the entire supply from the reactor to each transfer line. Thus, all control valves work in parallel, opening and closing with the balancing control used to balance the loads on the transfer lines.

[0048] Suspension loading on transfer lines can alternatively be determined by means of other measures previously described, such as flow / velocity or pressure drop. The entire supply to the transfer lines can be controlled directly on the basis of reactor pressure or on the basis of flow control.

The inactive heater must be activated when the selected indicator (selected indicators) of the heater performance indicates that at least one of the operating heaters operates at 95%, preferably 90% and most preferably at 80% or more of its maximum capacity. When two or more heaters operate in parallel, one of them must be turned off when at least one of them is operating at less than 60%, preferably less than 40% of the maximum capacity. A simple indicator of heater performance is the water heater control valve data that controls the amount of steam supplied to the jacket surrounding the heater and thereby the heat supply to the heater, therefore an additional inactive heater is preferably activated when the water vapor control valve of any significant heating section is open to the heater. more than 90%.

[0050] In both the single heater embodiments and the plurality of parallel heaters, the flow rate of the polymer containing polymer withdrawn from the polymerization reactor is preferably controlled using a pressure or flow control valve, most preferably placed between the particle concentration enhancer and the heater transfer line inlet. The control valve is designed such that it has a pressure drop between 45% and 90%, most preferably between 50% and 80% pressure drop between the reactor and the entrance to the first tank located below. The heated transfer line is preferably designed such that it has a pressure drop between 5% and 75%, most preferably between 10 and 35% of the pressure drop between the reactor and the entrance to the degassing tank.

[0051] The polymer-containing stream may comprise a vapor component as well as a liquid component. Typically, the vapor fraction of the fluid component of the stream containing polymers at the inlet of the heater varies from 5 to 60 mole%. In one preferred embodiment of the invention, a pressure or flow control valve is used above the heater, and the steam fraction of the stream at the inlet of the heater ranges between 25 and 60 mol%. The vapor fraction of the jet fluid component at the outlet of the heater may vary between 70 and 100 mole%, typically 95-100 mole%, most preferably greater than 99 mole%.

[0052] This invention can be used in any polymerization process (e.g., a gas phase, a slurry or solution) containing a polymeric stream that needs to be heated to evaporate the liquid while decreasing pressure.

[0053] The olefin copolymerization processes in the suspension phase are well known in the art. Such processes can be carried out, for example, by introducing a monomer and a co-monomer into a stirred reactor or a continuous loop reactor containing polyolefins and a polymerization catalyst. The reactor is usually controlled to achieve the desired melt index and density for the polymer at optimal production and temperature.

[0054] The processes for the production of polyethylene by suspension polymerization usually withdraw the polymer from the polymerization reactor together with a significant amount of liquid hydrocarbons, and therefore the invention is particularly important in such processes. The suspension in such reactors typically comprises a molecular polymer, a hydrocarbon solvent (s), a (co) monomer (s), a catalyst, a chain terminating agent such as hydrogen and other reactor additions. The suspension in particular contains 20-75, preferably 30-70 weight percent based on the total weight of the molecular polymer slurry and 80-25, preferably 70-30 weight percent based on the total weight of the suspending agent suspension, wherein the suspending agent is the sum of all liquid components in a reactor and containing a solvent, an olefin monomer and other additives; the solvent may be an inert solvent or it may be a reactive solvent such as a liquid olefin monomer. When the main solvent is a solvent, the neutral olefin monomer usually contains 220 wt.%, More specifically 4-10 wt.%. suspension.

[0055] The polymerization is usually carried out at temperatures in the range of 50-125 ° C and a pressure in the range of 1-100 bar. The catalyst used may be any catalyst conventionally used for the polymerization of olefins, such as chromium oxide, Ziegler-Natta catalyst or metallocene catalyst. The resulting suspension containing the polymer and solvent and in most cases the catalyst, olefin monomer and comonomer can be released periodically or continuously, optionally using concentration enhancing devices, such as hydrocyclones or depositionsides to minimize the amount of fluid withdrawn from the polymer.

[0056] This invention particularly relates to loop reactor polymerization, in which the suspension circulates in the reactor usually through the use of a pump or a stirrer. Full-loop liquid reactors are particularly well known in the art and are described, for example, in US Patents 3152872, US 3242150 and US 4613484. The loop reactor has a continuous tubular structure comprising at least two, e.g. four, vertical sections and at at least two, for example, four horizontal sections. The heat of polymerization is usually removed by indirect exchange with a cooling agent, preferably with water, in the coatings surrounding at least a portion of the tubular loop reactor. The volume of the loop reactor can vary but it usually ranges from 20 to 170 m³.

[0057] In commercial establishments, the molecular polymer is separated from the solvent in such a way that the solvent is not exposed to contaminants to allow the solvent to be recovered to the polymerization zone with minimal, if any, purification. Separation of the molecular polymer produced during the process of the invention from the solvent can usually take place using any method known in the art, which may for example include:

(i) the use of discontinuous vertical settling branches so that the flow of the suspension through the aperture provides a zone in which the polymer molecules can to some extent settle out of the solvent or (ii) continuous product withdrawal via single or multiple retraction ports whose location can be any on a loop reactor but usually they are located adjacent to the lower end of the horizontal loop section. Accordingly, the operation of large diameter reactors and high solids concentrations in the slurry minimizes the amount of main solvent withdrawn from the polymerization loop. The use of bracing devices on the withdrawn polymer suspension,

[0058] The withdrawn and preferably concentrated polymer suspension is usually subjected to pressure reduction prior to its flow through the heater according to the invention to the main flash separator.

[0059] The solvent and any monomer vapor recovered in the main flash separator are typically compacted, preferably without recompression, and reused in the polymerization process. The pressure in the main flash separator is usually controlled to allow condensation using an easily accessible cooling medium (e.g., cooling water) of substantially all of the vapors from the separator before recompression. The pressure in the flash separator is in the range of 2-25 bar, more typically 5-20 bar and most often 6-11 bar. The particulate material recovered from the main flash separator is typically fed to the secondary flash separator to remove the remaining volatiles. Alternatively, the slurry can be fed into a flash separator of lower pressure than in the above-mentioned main liquid-gas separator tank so that recompression for the condensation of the recovered solvent is required. The use of a high pressure flash separator is preferred.

[0060] An example of a type of polymerization process for which the invention is particularly useful is the continuous polymerization of olefins, preferably alpha-monoolefins, in the reaction zone, preferably being a closed loop of elongated tube. Olefin (s) are continuously added and come into contact with the catalyst in the hydrocarbon solvent. The monomer (s) polymerize to form a suspension of polymeric particulates suspended in

- polymerizing agent or solvent. The rate of retraction of the polymer product is controlled by the valve above the heater according to the invention.

[0061] The concentration of the solid particles in the suspension in the reactor is typically above 20% by volume, preferably about 30% by volume, e.g. 20-40% by volume, preferably 25-35% by volume, with the volume percentage being [(total volume of suspension - volume of suspending agent) / (total volume of suspension)] x100. The solids concentration, measured as a percentage by weight, which corresponds to a volume percent, will vary depending on the polymer produced, especially depending on the solvent used. When the polymer produced is polyethylene and the solvent is an alkane, e.g. isobutane, it is preferred if the solids concentration is above 30 wt.%, Especially above 40 wt.%, E.g. in the range of 40-60 wt.%, Preferably 45-55%. wt. based on the total weight of the suspension. It was found

[0062] This type of process can optionally be made in a multiple reactor system. The second or subsequent reactor from the multiple reactor system can be another loop reactor or any olefin polymerization reactor, e.g. a fluid bed reactor. However, usually the second or any subsequent reactor from the multiple reactor system is another loop reactor. Such multiple reactor systems can be used to make monomodal or multimodal polymers, preferably multimodal polymers.

[0063] For a number of reactors, the first reactor in the series is provided with a catalyst or prepolymer and optionally a cocatalyst as additive to the solvent and monomer, each subsequent reactor is provided with at least a monomer, especially ethylene, and a suspension formed in an earlier reactor in series this blend contains a catalyst and a blend of polymers made in the previous reactor from the series. It is also possible to provide a second reactor and / or, if appropriate, at least one of the subsequent reactors with fresh catalyst and / or cocatalyst. However, more often the catalyst and cocatalyst are introduced only into the first reactor.

[0064] In the case where the plant has at least two reactors in series, the polymer with the highest melting index and the polymer with the lowest melt index can be produced in two adjacent or non-adjacent reactors in series. Hydrogen is maintained (i) at low (or zero) concentration in the reactor (s) producing components of high molecular weight, e.g. a hydrogen percentage of between 0-0.1% vol. and (ii) at a very high concentration in a reactor (or reactors) producing components of low molecular weight, e.g. a hydrogen percentage of between 0.5-2.4% by volume. Reactors

- they can produce a polymer with substantially the same melt point in subsequent reactors.

[0065] When polymers reactor systems with molecular weights less than 50 kDalton or more than 150 kDalton have been produced, in the past particular problems have arisen in the clogging of the reactor and deposition in the heater between the polymerization reactor and the degassing tank. Such problems can be aggravated by the high concentration of particulates in the heater. This is another problem that can be solved by using a heater according to the invention.

EXAMPLE 1 [0066] A polymerization reactor polymerizing ethylene and a hexene comonomer in an isobutane solvent operates at a pressure of 41 bar and a temperature of 95 ° C. It releases a stream containing polymers in the form of a suspension, the liquid component of which contains 91% mol isobutane, 8% mol of unreacted ethylene and 1% mol of hexene. The solids content of the slurry is about 40% by weight, they contain polyethylene with a density of 940 kg / m³ and a certain amount of unused catalyst. These are the same conditions as in Example 1.

[0067] The slurry from the reactor flows through a pressure control valve to reduce the pressure before entering the heater according to the invention. The conditions at the inlet to the heater are as follows:

Temperature: 82.4 ° C

Pi pressure: 17.4 bar

Speed ​​Vi: 10.7 ms^-1

Reynolds number: 1.72 million

A solids concentration of 40% by weight; the remaining liquid phase, 40% by weight vapor and 60 wt.% liquid.

[0068] The heater is 187 m long and comprises 3 straight and vertical sections, each 58 m long, connected by means of 180 ° bends; elbows together constitute 13 m of total length and are not heated. All sections have an inner diameter of 78 mm and a wall thickness of 5.5 mm, which gives an L / Dave value of about 2397. The thermal conductivity of the entire pipe is 46.4 W / mK. The heating element has the form of concentric outer pipes extending along each 26 m long section through which cooled superheated steam passes.

[0069] The slurry flows through the heater at a rate of 15 tons / hour. The length and diameter of the tube, the heat input in the heater from the heating medium to the slurry and the speed and initial solids content of the slurry are calculated to ensure that the heat supply to the slurry as it flows through the heater is sufficient to completely evaporate the liquid phase until the slurry emerges from the heater. The suspension flows out of the heater at a temperature of 76 ° C, after a pressure equal to 9 bar

- 16 Vo speeds equal to 63.3 m / s (Vo / Vi = 5.9), with a Reynolds number of 3.3 million. This equals the pressure drop on the heater equal to 0.045 bar / m. At this pressure, the vapor dew point would be around 60.8 ° C, so the temperature of the stream exceeds the dew point by 15 ° C and the liquid is completely evaporated without the risk of condensation when it emerges from the heater.

[0070] The temperature of the inner wall of the heater is between 89 ° C and 93 ° C over the entire length of the heater compared to the softening temperature of the polymer of about 128 ° C. The heat exchange coefficient from steam to the suspension on the heater wall is calculated as 984 W / m²K.

EXAMPLE 2 [0071] In this Example, the heater has a lower L / Dave ratio but also an increasing diameter.

[0072] The polymerization reactor polymerizing ethylene and the hexene comonomer in the isobutane solvent works at a pressure of 40 bar and a temperature of 95 ° C. It releases a polymer-containing stream which is in the form of a suspension, the liquid component of which contains 91% mol isobutane, 8% mol of unreacted ethylene and 1% mol of hexene. The solids content of the suspension is about 40 wt.%, Containing polyethylene with a density of 940 kg / m³ and a certain amount of unused catalyst.

[0073] The slurry from the reactor first flows through the hydrocyclone for a solids concentration of up to 50% by weight. and then a pressure control valve to reduce the pressure before entering the heater according to the invention. The conditions at the inlet to the heater are as follows:

Temperature: 76 ° C

Pi pressure: 14.4 bar

Speed ​​Vi: 16.6 ms-1

Reynolds number: 2 million

A solids concentration of 50% by weight; the remaining liquid phase, 40% by weight vapor and 60 wt.% liquid.

[0074] The heater has a length of 152 m and comprises 5 straight and vertical sections, each of them 26 m long, which are connected by means of 180 ° bends; elbows together constitute 22m of total length. Each of the first three sections has an internal diameter of 78mm, while each of the other two sections has an inner diameter of 102mm. A single diameter increase takes place at the exit from the elbow connecting the third and fourth segments. Thus, the heater has a Do / Di value of 1.33 and an L / Dave value of about 1730. The wall thickness of the tube diameter 78mm is 5.5mm, and the wall thickness of the pipe with a diameter of 102mm is 6.0mm. As in Example 1, the thermal conductivity of the entire pipe wall is 46.4 W / mK, and the heating element has the form of concentric outer pipes extending along each of the straight 58 m long sections through which

- 17 cooled chilled superheated steam.

[0075] The slurry flows through the heater at a rate of 20 tons / hour. The length and diameter of the tube, the heat input from the heater and the speed and initial solids content of the slurry are calculated to ensure that the heat supply to the slurry as it flows through the heater is sufficient to completely evaporate the liquid phase until the slurry leaves the heater. The suspension comes out of the heater at a temperature of 80 ° C, a pressure of 10 bar, a Vo speed of 30 m / s (Vo / Vi = 1.78) and a Reynolds number of 2.7 million. This means a pressure drop on the heater of 0.03 bar / m. At this pressure, the vapor dew point is around 65 ° C, so the stream temperature exceeds the dew point by 15 ° C and the liquid is completely evaporated without the risk of condensation when it leaves the heater.

[0076] The temperature of the inner wall of the heater is between 89 ° C and 93 ° C over the entire length of the heater; for comparison, the polymer softening temperature is about 128 ° C. The coefficient of heat exchange from steam to the suspension on the wall of the heater is calculated at 600 W / m²K.

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